The present invention relates to a spindle motor, particularly to an apparatus for preventing an overrun of the spindle motor due to a defective speed control apparatus.
The spindle motor needs to be rotated at constant speed in video disk players, consequently a frequency generator has been used in order to control the speed of the spindle motor. FIG. 1 shows a block diagram of a conventional speed control apparatus of a spindle motor. In a motor driving means 100 for driving the spindle motor 1, a power supply 12 is connected to the spindle motor 1 to provide electric power for a rotation of the spindle motor 1. A disk 2 is placed at the top of a rotary axis 4 of the spindle motor in order to be rotated according to the rotation of the spindle motor 1.
White and black stripes (not shown) are formed in said spindle motor axis 4, so that the motor speed controlling means 200 can detect the frequency signal effected by the change of the white and black stripes as the motor rotates.
A motor speed controlling means 200 controls a rotating speed of said spindle motor 1, in which a frequency sensor 3 detecting a frequency signal according to a rotation of the spindle motor axis 4, and a reference frequency synchronization generator 6 changing to provide a reference frequency according to a control signal microcomputer MICOM 7 are connected to a time-base controller 5 providing a control signal by comparing the applied frequency signals. Said time-base controller 5 is connected to a spindle servo 8 which controls the rotating speed of said spindle motor 1 according to an applied control signal.
An image processing device 300 which picks up the data signal of the disk 2 and provides it as an image signal consists of a light pickup 9 to read the recorded data from the disk 2, a preamplifier 10 to amplify the signal applied from the light pickup 9, and a video signal processor 11 to change the amplified data signal to the image signal.
In the conventional speed control apparatus, when the spindle motor 1 is driven by the power supply 12, the frequency sensor 3 detects the frequency signal generated by the white and black stripes on the axle 4 of the spindle motor 1 and provides it to the time-base controller 5.
Simultaneously, the reference frequency generator 6 provides a reference frequency signal to the time-base controller 5. According the control signal of the MICOM 7, the time-base controller 5 compares the detected frequency signal with the reference frequency signal and provides a control signal based on the compared result to the spindle servo 8. The spindle servo 8 controls the speed of the spindle motor 1 according to the control signal provided by the time-base controller 5. The light pickup 9 reads the data from the disk 2 and applies it to the preamplifier 10, so that an read data is amplified and is changed to the image signal by the video processor 11.
However, in the conventional speed control apparatus described above, if the frequency sensor 3 is defective, the detected frequency signal, which is compared with the reference frequency signal, is no longer applied to a time-base controller 5. Thus, a control signal to increase the speed of the spindle motor 1 is continuously applied to the spindle servo 8, thereby continuously 1 increasing more and more the speed of the spindle motor 1. In the worst case, the video disk player can be broken.